1. Field of the Invention
The invention relates to an offset voltage eliminating circuit structure for a protection mechanism of a dimmer, and more particularly, to a circuit that provides a current output when connected to an external resistive dimmer and eliminates the offset voltage created by an external limiting resistor used for circuit protection.
2. Description of the Prior Art
LEDs are widely used in various lighting conditions due to their characteristics such as low energy consumption and long life. However, there are considerable differences in the physical characteristics between LEDs and traditional light elements, such that a dimmer circuit suitable for a traditional light element may not be used directly to control the brightness of a LED. As such, one possible approach is to connect a converting circuit between the LED and the traditional dimmer circuit in order to turn the control mechanism of the traditional dimmer circuit into a control voltage for controlling the LED.
Traditional light dimmers can generally be classified as voltage, resistive and PWM types. Regardless of which type it is, an analog dimming signal from 0 to 10V is usually output. This voltage represents brightness from 0% to 100%. The above converting circuit must be able to accept the analog dimming signal (0 to 10V) output by the traditional dimmer and convert it into a control signal that is variable between 0% and 100% in order to drive an appended power supply to provide different voltages to the LED, thus allowing the LED to have different brightness.
As shown in FIG. 1, a converting circuit that can be connected with a traditional dimmer is shown. The converting circuit mainly includes: a constant current source A and a hysteresis comparator CMP1, wherein the constant current source A is formed of a cascaded current mirror circuit consisting of symmetric P-type transistors MP1, MP2, MP3 and MP4 and an N-type transistor MN1, a first operational amplifier OPA1 and a first resistor R1. A voltage V1 can be input into the non-inverting input of the first operational amplifier OPA1. The inverting input of the first operational amplifier OPA1 is connected to the gate of the N-type transistor MN1 and grounded via the first resistor R1. The source of the P-type transistor MP4 is connected to an output Pout. The output Pout is also connected to a dimming signal input ADJ_in.
The non-inverting input of the hysteresis comparator CMP1 is connected to the output Pout (i.e. the dimming signal input ADJ_in), while the inverting input is input with a saw-tooth wave.
Referring to FIG. 2, when the above circuit is in actual use, if the dimming signal input ADJ_in is connected with an output of a traditional resistive dimmer (which can be regarded as a variable resistor RT), then the constant current source A is controlled to output a current I=V1/R1. The current flows through the variable resistor RT to create a voltage VT=RT*V1/R1. This voltage VT can be varied by adjusting the resistance of the variable resistor RT, and it can be input to the hysteresis comparator CMP1 through the output Pout. The hysteresis comparator CMP1 then compares it with the saw-tooth wave to output a GD pulse for adjusting the brightness of the LED. The amount of duty cycle of the GD pulse is proportional to VT.
Referring now to FIG. 3, when the above circuit is in actual use, if the dimming signal input ADJ_in is connected with an output of a traditional voltage-type dimmer (which can be regarded as a DC voltage VA), then the DC voltage VA can be input into the hysteresis comparator CMP1 via the output Pout. The hysteresis comparator CMP1 then compares the DC voltage VA with the saw-tooth wave and outputs a GD pulse for adjusting the brightness of the LED.
Referring now to FIG. 4, when the above circuit is in actual use, if the dimming signal input ADJ_in is connected with an output of a traditional PWM-type dimmer, then a resistor Rf can be provided between the dimming signal input ADJ_in and the output Pout, and the output Pout is further connected to ground via a capacitor C, thus forming a RC oscillating circuit using the resistor Rf and the capacitor C. The PWM signal creates an effective DC voltage VA1 on the output Pout through the discharging of the capacitor C. The voltage VA1 can be input into the hysteresis comparator CMP1 via the output Pout. The hysteresis comparator CMP1 then compares the DC voltage VA with the saw-tooth wave and outputs a GD pulse for adjusting the brightness of the LED.
However, in actual practice, a current-limiting resistor is provided on the dimming signal input ADJ_in to protect the constant current source A and associated circuits from damages caused by high voltage or reversed voltage polarities. Therefore, under normal use (this is especially the case for resistive dimmers, in which the control voltage is formed across a variable resistor RT by outputting a current from the constant current source A), any current that flows through the current-limiting resistor will inevitably create a voltage drop. When this voltage drop of the current-limiting resistor is added with the analog dimming signal input by the dimming signal input ADJ_in, it will cause a voltage offset on the analog dimming signal. This voltage offset will then affect the dimming control voltage output by the hysteresis comparator CMP1, so it must be eliminated in order to obtain the required analog dimming signal input.
In view of the shortcomings in the conventional converting circuit when in use, the present invention is proposed to provide improvements that address these shortcomings.